The present invention generally relates to lamp reflectors and methods for making such reflectors. Particularly, the reflectors are made from a thermally conductive polymer composition that can dissipate heat from a heat-generating light source within the reflector. The reflectors can be used in automotive headlamps, flashlights, and other lighting fixtures.
In the past, reflector housings for automotive headlamps and other lighting devices were made by stamping sheets of metal into a desired shape. A layer of aluminum was vacuum-deposited onto the shaped metal to form a highly polished reflective surface. This metal stamping process produced headlamps having good mechanical strength, but only a limited number of simple shapes could be made. As designs for automobile headlights changed, the need for reflectors having more complex aerodynamic structures grew.
Today, reflector housings for automotive headlamps are often made from thermosetting or thermoplastic compositions that can be molded into a variety of shapes. Typically, these compositions contain a resin and a reinforcing material that improves the strength and dimensional stability of the molded housing.
For example, Weber, U.S. Pat. No. 5,916,496 discloses a method of molding a vehicle lamp reflector from a composition containing substantial amounts of fiber and mineral fillers. The method produces a lamp reflector having a substantially organic skin over a substantially inorganic core. A layer of aluminum can be vacuum-deposited onto the organic skin without using a base coat.
Baciu et al., U.S. Pat. No. 4,617,618 discloses a headlamp reflector made by a co-injection molding process. The core of the reflector is made from a composition containing polyalkylene terephthalate and hematite (85 to 95% by weight of Fe2O3) particles having a particle size less than 70 xcexcm. Glass fibers, microbeads, and other filler materials can be added to the composition.
Withoos et al., U.S. Pat. No. 4,188,358 disclose a method of manufacturing a metallized plastic reflector. A film or fabric of fibrous material (for example, glass or carbon fibers) is provided over a convex surface of a mold and saturated with a thermo-hardening synthetic resin. After partial hardening of the resin, a layer of liquid metal particles is sprayed onto the resin. A supporting layer including a synthetic resin reinforced with fibrous material (for example, polyester or nylon) is provided over the metal layer.
The light sources in automotive headlamps and other reflector devices can generate a tremendous amount of heat. These devices must meet maintain an operating temperature within the enclosed reflective region (area between the reflector and lens assembly) of no greater than 190xc2x0 C. Many reflector devices are made from molded plastics that are poor conductors of heat. As a result, heat remains trapped within this reflective area, and temperatures can quickly rise above 190xc2x0 C. This overheating phenomenon often occurs in underwater flashlights where the entire lighting structure is made of plastic and sealed to prevent infiltration of water.
The industry has attempted to solve these overheating problems by a variety of ways. One process involves molding large milled aluminum heat sinks onto the back of automotive headlamp reflectors. These heat sinks are used often with heat pipes to transfer heat from the back of the reflector to other heat sinks remotely located in the assembly. Another process involves making reflectors from sheets of metal. For example, a sheet of aluminum can be milled or spun into the desired shape of the reflector. However, these manufacturing processes are costly, and it can be cumbersome to produce reflectors having complex shapes using such processes.
There is a need for a thermally conductive lamp reflector that can effectively remove heat from heat-generating lamp assemblies such as automotive headlamps, underwater flashlights, and the like. The present invention provides such a thermally conductive reflector.
This invention relates to a thermally conductive lamp reflector including a shell having a surface that is coated with a metallized reflective layer. The shell is made from a composition containing a base polymer matrix and thermally conductive filler material. The surface of the shell can be metallized with a layer of aluminum. A protective layer comprising polysiloxane, silicon dioxide, or acrylic resin can be coated over the aluminum-coated layer. The reflector has a thermal conductivity of greater than 3 W/mxc2x0 K and more preferably greater than 22 W/mxc2x0 K.
A thermoplastic polymer selected from the group consisting of polycarbonate, polyethylene, polypropylene, acrylics, vinyls, and fluorocarbons can be used to form the matrix. Preferably, a liquid crystal polymer is used. Alternatively, thermosetting polymers such as elastomers, epoxies, polyesters, polyimides, and acrylonitriles can be used. The filler material may be selected from the group consisting of aluminum, alumina, copper, magnesium, brass, carbon, silicon nitride, aluminum nitride, boron nitride, zinc oxide, glass, mica, and graphite. The filler material may be in the form of particles, fibers, or any other suitable form. The polymer matrix preferably constitutes about 30 to about 80% and the thermally conductive filler preferably constitutes about 20 to about 70% by volume of the composition.
In one embodiment, the composition includes: i) about 30 to about 60% by volume of a polymer matrix; ii) about 25 to about 60% by volume of a first thermally conductive filler material having an aspect ratio of 10:1 or greater; and (iii) about 10 to about 15% by volume of a second thermally conductive filler material having an aspect ratio of 5:1 or less.
The present invention also encompasses methods for making thermally conductive lamp reflectors.